ANNIVERSARY ADDRESS. 15 
and with Mr. Spottiswoode, both of whom had very fine instru- 
ments. That belonging to Mr. Spottiswoode was, I understood, 
the largest in the world, the diameter of the prisms being about 
4 inches. He informed me that the supply of Iceland spar fit for 
making prisms had entirely stopped, the quarry being exhausted. 
He had tried to induce people to search for other deposits in 
Iceland, but without effect, and he advised me to secure the best 
prisms available without delay, as the market would soon be 
cleared. He said he believed there was only one large crystal in 
London remaining to be worked up; that there was a larger one 
at Copenhagen, but the quality was not so good; and that if I 
did not secure the English one it would likely soon go to America. 
It was some time before I could learn where this big crystal was 
to be found. There seemed a degree of mystery about it. At 
length I came upon the coveted prize in an obscure and dingy 
workshop, and speedily agreed upon terms for its purchase. It 
made a polariser of 2? inches aperture, but it was impossible to 
get an analyser of the same size, and I had to be content with one 
less than 2 inches. Now, however, the University is in possession 
of a fine instrument, there being only (as I was assured) three or 
four polarisers in existence of larger aperture. 
- Another instrument of far less pretension, in which I felt con- 
siderable interest, was the “ Rainband Spectroscope.” It is simply 
a direct-vision pocket spectroscope, carefully made, so as to give 
fine definition of the absorption lines. It was brought into 
prominence chiefly through the writings of Professor Piazzi 
Smyth, Astronomer Royal for Scotland, who extolled its value in 
prognosticating rain. One of these instruments, made by A. 
Hilger, I now exhibit. I have made frequent use of it for more 
than a year, and only on one occasion has it afforded me a distinct 
indication of rain prior to the usual atmospheric signs. This was 
at Melbournc, at the close of a day of hot wind, when the air 
seemed excessively dry, and no appearance of a change. Indeed, 
ordinary observers were prognosticating a hotter wind for next 
day ; but this little instrument showed the vapour bands with 
